Vertical bipolar transistor with a majority carrier accumulation layer as a subcollector for SOI BiCMOS with reduced buried oxide thickness for low-substrate bias operation

ABSTRACT

The present invention provides a “subcollector-less” silicon-on-insulator (SOI) bipolar junction transistor (BJT) that has no impurity-doped subcollector. Instead, the inventive vertical SOI BJT uses a back gate-induced, majority carrier accumulation layer as the subcollector when it operates. The SOI substrate is biased such that the accumulation layer is formed at the bottom of the first semiconductor layer. The advantage of such a device is its CMOS-like process. Therefore, the integration scheme can be simplified and the manufacturing cost can be significantly reduced. The present invention also provides a method of fabricating BJTs on selected areas of a very thin BOX using a conventional SOI starting wafer with a thick BOX. The reduced BOX thickness underneath the bipolar devices allows for a significantly reduced substrate bias compatible with the CMOS to be applied while maintaining the advantages of a thick BOX underneath the CMOS. A back-gated CMOS device is also provided.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/931,855, filed Sep. 1, 2004.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device, and moreparticularly a vertical bipolar transistor that is formed usingsilicon-on-insulator (SOI) integrated bipolar transistor andcomplementary metal oxide semiconductor (hereinafter BiCMOS) technology.

BACKGROUND OF THE INVENTION

The semiconductor industry has been seeking more cost effectivesolutions for manufacturing BiCMOS devices for mass applications ofradio frequency (RF)/analog and wireless/fiber-based telecommunicationsfor decades. Si/SiGe BiCMOS technology is widely used and has been quitesuccessful. However, as complementary metal oxide semiconductor (CMOS)adopts thin silicon-on-insulator (SOI) substrates for lower power andhigher speed (due to device scaling), the thick subcollector ofconventional bipolar junction transistors (BJTs) becomes incompatiblewith the integration of high-performance SOI CMOS devices.

In order to facilitate integration with SOI CMOS, lateral SOI BJTs havebeen proposed and studied. See, for example, S. Parke, et al. “Aversatile, SOI CMOS technology with complementary lateral BJT's”, IEDM,1992, Technical Digest, 13-16 Dec. 1992, page(s) 453-456; V. M. C. Chen,“A low thermal budget, fully self-aligned lateral BJT on thin film SOIsubstrate for lower power BiCMOS applications”, VLSI Technology, 1995.Digest of Technical Papers. 1995 Symposium on VLSI Technology, 6-8 Jun.1995, page(s) 133-134; T. Shino, et al. “A 31 GHz finax lateral BJT onSOI using self-aligned external base formation technology”, ElectronDevices Meeting, 1998. IEDM '98 Technical Digest, International, 6-9Dec. 1998, page(s) 953-956; T. Yamada, et al. “A novel high-performancelateral BJT on SOI with metal-backed single-silicon external base forlow-power/low-cost RF applications”, Bipolar/BiCMOS Circuits andTechnology Meeting, 1999. Proceedings of the 1999, 1999, page(s)129-132; and T. Shino, et al. “Analysis on High-FrequencyCharacteristics of SOI Lateral BJTs with Self-Aligned External Base for2-GHz RF Applications”, IEEE, TED, vol. 49, No. 3, pp. 414, 2002.

Even though lateral SOI BJT devices are easier to integrate with SOICMOS, the performance of such devices is quite limited. This is becausethe base width in the lateral SOI BJTs is determined by lithography.Hence, it cannot be scaled down (less than 30 nm) readily without moreadvanced and more expensive lithography technologies such as e-beamlithography.

Another type of SOI BJT, which is a vertical SOI SiGe bipolar device,has also been proposed and demonstrated to offer higher base-collectorbreakdown voltage, higher early voltage and better BVCEO-fT tradeoff.This type of SOI BJT is described, for example, in J. Cai, et al.,“Vertical SiGe-Base Bipolar Transistors on CMOS-Compatible SOISubstrate”, 2003 IEEE Bipolar/BiCMOS Circuits and Technology Meeting.This SOI BJT device uses a fully depleted SOI layer as the collector atzero substrate bias. The application of a substrate bias to this SOI BJTdevice allows for significant improvement in overall device performanceby reducing collector space-charge region transit time and collectorresistance through the formation of an accumulation layer.

A problem with the SOI BJT device described above is that the buriedoxide (BOX) layer in high performance CMOS SOI substrates is typically100-200 nm thick. As a result, the substrate bias needed for significantperformance improvement is unacceptably large (greater than about 20 V).In order for these devices to be practical for SOI BiCMOS applications,the substrate bias must be held at or below the voltage applied to theCMOS, typically less than 3 V.

In view of the above, there is a need for providing a SOI BJT structurethat overcomes the drawbacks mentioned in the prior art SOI BJTs.

SUMMARY OF THE INVENTION

The present invention provides a vertical SOI BJT which uses a SOI layerwith a back gate-induced majority carrier accumulation layer as asubcollector located on regions of a second buried insulating regionhaving a second thickness using a standard SOI starting wafer with afirst buried insulating region having a first thickness and the methodthereof. In accordance with the present invention, the first thicknessof the first buried insulating region is greater than the secondthickness of the second buried insulating region. The reduced thicknessof the second buried insulating region underneath the bipolar devicesallows for a significantly reduced substrate bias that is CMOScompatible, while maintaining the advantages of the thick first buriedinsulating region underneath the CMOS.

The accumulation layer can then be formed to reduce collector resistanceand transit time by applying a back-bias that will not compromise thequality and reliability of the CMOS.

A method of forming a bipolar transistor including a localized thinburied insulating region (second buried insulating region) is provided.In broad terms, the method of the present invention includes the stepsof:

providing a silicon-on-insulator (SOI) substrate comprising a firstsemiconductor layer containing a first conductivity type dopant locatedover a first buried insulating layer, wherein a portion of the firstburied insulating layer beneath said first semiconductor layer isremoved providing an undercut region;

forming a second buried insulating layer on exposed surfaces of saidfirst semiconductor layer, wherein said second buried insulating layeris thinner than said first buried insulating layer;

filling the undercut region and the removed portion of the firstsemiconductor layer with a conductive back electrode material;

forming a base comprising a second semiconductor layer containing asecond conductivity type dopant that is different than the firstconductivity type dopant on said substrate;

forming an emitter comprising a third semiconductor layer including saidfirst conductivity type dopant over a portion of said base; and

biasing the conductive back electrode material to form an accumulationlayer at an interface between the first semiconductor layer and thesecond buried insulating layer.

The first semiconductor layer includes an intrinsic collector and anextrinsic collector. The base may include a single crystal portion atopsemiconductor material, and a polycrystalline portion atop insulatingmaterial.

In accordance with the present invention, a vertical NPN or PNP SOI BJTcan be formed. The NPN transistor is formed when the first and thirdsemiconductor layers contain an n-type dopant, while the secondsemiconductor layer comprises a p-type dopant. A PNP transistor isformed when the first and third semiconductor layers contain a p-typedopant and the second semiconductor layer contains an n-type dopant.

Specifically, a trench is first etched through the first semiconductorlayer of an SOI substrate exposing the first buried insulating layerwhich normally has a thickness from about 100 to about 500 nm. A portionof the first buried insulating layer is then removed using an isotropicetch process that undercuts the first semiconductor layer. A thininsulating layer (less than about 15 nm) is then grown to form thesecond buried insulating layer. The trench and area where the firstburied insulating layer was removed is filled in with a conductivematerial such as in-situ doped polysilicon. The conductive-fill can thenbe used to apply a substrate bias. These processing steps provide astructure that includes a conductive back electrode that contains asecond buried insulating region of a second thickness located on asurface thereof and a first buried insulating region of a firstthickness that is greater than the second thickness that is locatedabutting the region containing the conductive back electrode and theoverlayer first insulating layer. In accordance with the presentinvention, a bipolar device can be formed atop this structure such thatit is located above the second buried insulating layer.

With such a reduced buried insulating layer thickness underneath thebipolar device, a significantly reduced substrate bias (less than 3 V)compatible with the CMOS is able to create a strong enough verticalelectric field to form an accumulation layer which forms thesubcollector of the inventive device, while maintaining the advantagesof a thick first buried insulating layer underneath the CMOS.

There are no known alternative solutions to this problem. One possiblealternative is to use a patterning process to form regions of thin andthick buried insulating regions on the SOI wafer during a SIMOX(separation by implantation of oxygen) process. However, by using anoxygen implant, it is difficult to make a buried insulating regionhaving a thickness of less than 10 nm. Moreover, it is difficult tocontrol the thickness of the buried insulating region formed byconventional SIMOX processes. Also, this method purposed above wouldrequire costly additional lithography and implant steps to produce theSOI wafers.

In addition to the method described above, the present invention alsocontemplates the bipolar transistor that is formed utilizing the abovemethod. Specifically, and in broad terms, the bipolar transistor of thepresent invention comprises:

a conductive back electrode for receiving a bias voltage;

a second buried insulating layer located over said conductive backelectrode having a second thickness;

a first buried insulating layer located adjacent to said second buriedinsulating layer and said conductive back electrode, said first buriedinsulating layer having a first thickness that is greater than thesecond thickness;

a first semiconductor layer located predominately over said secondburied insulating layer, said first semiconductor layer including afirst conductivity type dopant, wherein said conductive back electrodeis biased to form an accumulation layer in said first semiconductorlayer at an interface between said first semiconductor layer and saidsecond buried insulating layer;

a base located atop at least said first semiconductor layer, said basecomprising a second semiconductor layer having a second conductivitytype dopant that differs from the first conductivity type dopant; and

an emitter comprising a third semiconductor layer of the firstconductivity type dopant located over a portion of said base.

In addition to SOI BJT's the present invention, in particularly thesubstrate including different regions of buried insulating thicknesscould be used as a substrate for forming a back-gated complementarymetal oxide semiconductor (CMOS) device. The back-gated CMOS devicecould be formed alone on the substrate or it could be formed with abipolar transistor, including the SOI HBT described above, in BiCMOSapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are pictorial representations (through different views)illustrating a single-finger emitter device of the present inventionalong two directions that are perpendicular to each other.

FIGS. 2A-2E illustrate the process flow for making the substrate that isemployed in the present invention which includes a second buriedinsulating region that is thin and an adjoining first buried insulatingregion that has a thickness that is greater than the second buriedinsulating region. The second buried insulating region is located atop aconductive back electrode.

FIGS. 3A-3B show cross sections of an SOI wafer that underwent theprocess illustrated in FIGS. 2A-2E. In the drawings, the first buriedinsulating layer was undercut by 0.3 microns. An 8 nm thick thermaloxide, e.g., the second buried insulating layer, was then grown followedby LPCVD polysilicon fill to form the conductive back electrode.

FIG. 4 is a cross sectional view illustrating an expanded view of thestructure shown in FIG. 2E.

FIGS. 5A and 5B are pictorial representations (through different views)illustrating a back-gated CMOS device of the present invention along twodirections that are perpendicular to each other.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a vertical SOI BJT which uses aSOI layer with a back gate-induced accumulation layer as thesubcollector located on regions of a second buried insulating regionhaving a second thickness using a standard SOI starting wafer with afirst buried insulating region having a first thickness and the methodthereof, will now be described in greater detail by referring to thedrawings that accompany the present application. The drawings areprovided for illustrative purposes and thus they are not drawn to scale.Moreover, in the drawings like and/or corresponding elements arereferred to by like reference numerals.

The present invention provides a bipolar transistor structure thatincludes a conductive back electrode for receiving a bias voltage, asecond buried insulating layer located over the conductive backelectrode, and a first semiconductor layer, which comprises an SOI layerof a SOI substrate, located over the second buried insulating layer. Thefirst semiconductor layer includes a collector region containing a firstconductive type dopant. The collector region includes an intrinsiccollector and an extrinsic collector. The extrinsic collector and theintrinsic collector, which are of the first conductivity type, havedifferent dopant concentration; the extrinsic collector having a higherdopant concentration that the intrinsic collector.

In accordance with the present invention, a base comprising a secondsemiconductor layer containing a second conductivity type dopant islocated atop the first semiconductor layer. The inventive bipolartransistor also includes an emitter comprising a third semiconductorlayer containing the first conductivity type dopant located over aportion of the base, e.g., the second semiconductor layer. Duringoperation, the conductive back electrode is biased to form anaccumulation layer in the SOI layer at an interface between the SOIlayer and the second buried insulating layer. The configuration of theinventive bipolar transistor structure will become more apparent byreferring to FIGS. 1A-1B.

One possible device layout of the inventive bipolar transistor is shownin FIGS. 1A-1B wherein a single-finger emitter device is shown. By“finger”, it is meant that the emitter has at least one portion thatextends outward from a common emitter region. Although the drawings showa one-finger emitter device, the present invention is not limited toonly that device layout. Instead, the present invention contemplatesdevice layouts that include a number of emitter-fingers. Multi-fingerconfigurations are preferred over the singe-finger device layout sincethey typically reduce the emitter resistance for achieving high ƒ_(max).i.e., the maximum oscillation frequency at which the unilateral powergain becomes unity.

The cross sectional views of the single-finger emitter device layout isshown in FIGS. 1A and 1B. FIG. 1A is the cross sectional view along anaxis B-B′, while FIG. 1C is the cross sectional view along an axis C—C′;the two axis are perpendicular to each other. Specifically, the crosssectional views shown in FIG. 1A and FIG. 1B depict a vertical bipolartransistor 10 of the present invention. The vertical bipolar transistor10 includes a Si-containing substrate layer 14, a first buriedinsulating layer 16 having a first thickness, a second buried insulatinglayer 22 having a second thickness that is less than the first thicknessof the first buried insulating layer 16. As shown, the first buriedinsulating layer 16 is located on an upper surface of the Si-containingsubstrate 14 and the second buried insulating layer 22 is located aroundthe conductive back electrode 24. The second buried insulating layer 22thus includes an upper portion 22 u located atop the conductive backelectrode 24 and a lower portion 221 located atop the Si-containingsubstrate 14. The upper portion 22 u of the second buried insulatinglayer 22 is the region in which the accumulation layer will formthereon.

The vertical bipolar transistor 10 shown in FIGS. 1A-1B further includestrench isolation regions 28 that are located, as shown in FIG. 1A, atopthe first buried insulating layer 16, as well as atop the conductiveback electrode 24, as shown in FIG. 1B. Hence, the trench isolationregions 28 surround the active device region of the structure. Thestructure also includes a first semiconductor layer 18 (hereinafterreferred to as the SOI layer) which is located on the upper portion 22 uof the second buried insulating layer 22 as well as a portion of thefirst buried insulating layer 16. The first semiconductor layer 18 isthe original SOI layer of the initial substrate employed in the presentinvention.

In accordance with the present invention, the first semiconductor layer18 is the collector region of the inventive structure that is doped witha first conductivity type dopant, either an n- or p-type dopant. Thecollector region includes an extrinsic collector 41 and an extrinsiccollector 43 that has a greater dopant density, i.e., concentration, ascompared to intrinsic collector 41. As shown, the intrinsic collector 41is located between two extrinsic collectors 43.

A base (or base region) 100 is located atop the SOI layer 18 and thetrench isolation region 28. The base 100 comprises a secondsemiconductor layer of a second conductivity type dopant that differs interms of its conductivity from the first conductivity type dopant. Thebase 100 comprises a polycrystalline portion 100 b and a singlecrystalline portion 100 a. As shown, the polycrystalline portion 100 bis located predominately atop isolation regions, while the singlecrystal portion 100 a is located atop the SOI layer 18.

Atop of the base 100 is an emitter 52 which is comprised of a thirdsemiconductor layer. The third semiconductor layer forming emitter 52may be comprised of the same or different material as the base 100 orthe SOI layer 18. The emitter 52 is heavily doped with the firstconductivity type dopant. First insulator 30 and second insulator 36 arelocated in the structure as well. The first insulator 30 is located atopthe structure including the SOI layer 18 and it has an opening thereinin which the intrinsic portion 100 a of the base 100 is in contact withthe SOI layer 18. The second insulator 36 is located atop portions ofthe base 100 and it also has an opening therein that allows the emitter52 to be in contact with the intrinsic portion 100 a of the base 100.

Although not shown, the exposed portions of the emitter 52, the SOIlayer 18, the polycrystalline region 100 b and conductive back electrode24 may include a metal silicide. The metal silicide located atop theexposed surfaces of the conductive back electrode 24 is the region inwhich biasing of the substrate can take place. During biasing, a portionof the SOI layer 18 that is located atop the upper portion 22 u of thesecond buried insulating region 22 is converted into an accumulationlayer 62. The accumulation layer 62 is a majority carrier layer thatserves as the subcollector of the inventive bipolar transistor. This isunlike prior art bipolar transistor in which the subcollector iscomprised of an impurity-doped region.

The process flow for making a substrate that includes the buriedinsulating regions of different thicknesses is illustrated in FIGS.2A-2E. It is noted that the process shown and described in making thesubstrate shown is similar to the process disclosed in co-pending andco-assigned U.S. patent application Ser. No. 10/787,002, filed Feb. 25,2004, the entire content of which is incorporated herein by reference.In the following description of the first and second buried insulatinglayers (or regions) 16 and 22, respectively, are referred to as buriedoxide (BOX) regions. Although BOX regions are depicted and described asoxides, the present invention works equally well when the regions 16 and22 are other insulating materials, i.e., nitrides or oxynitrides.

FIG. 2A shows the cross-section of a typical SOI substrate 12 used for ahigh-performance CMOS application that can be employed in the presentinvention. The initial SOI substrate 12 comprises a Si-containingsubstrate layer 14, a first buried insulating layer 16 of a firstthickness (herein after thick BOX) 16, and a top Si-containing layer 18(which is, in accordance with the nomenclature of the present invention,the first semiconductor layer or the SOI layer 18). The term“Si-containing” is used herein to denote any semiconductor material thatincludes silicon therein.

Illustrative examples of such Si-containing materials include but arenot limited to: Si, SiGe, SiGeC, SiC, Si/Si, Si/SiGe, preformed SOIwafers, silicon germanium-on-insulators (SGOI) and other likesemiconductor materials.

The SOI layer 18 of the initial SOI substrate 12 is typically a dopedlayer, which may contain an n- or p-type dopant. Doping can beintroduced into the SOI layer 18 prior to, or after formation of the SOIsubstrate 12. The doped SOI layer 18 comprises the collector region ofthe inventive bipolar transistor 10. The dopant concentration within theSOI layer 18 is typically from about 1E17 to about 1E19 atoms/cm³.

The Si-containing layer 18 of the SOI substrate 12 may have a variablethickness, which is dependent on the technique that is used in formingthe SOI substrate 12. Typically, however, the Si-containing layer 18 ofthe SOI substrate 12 has a thickness from about 10 to about 1000 nm,with a thickness from about 50 to about 500 nm being more typical. Thethickness of the thick BOX 16 may also vary depending upon the techniqueused in fabricating the SOI substrate 12. Typically, however, the thickBOX 16 of the present invention has a thickness from about 100 to about1000 nm, with a BOX thickness from about 120 to about 200 nm being moretypical. The thickness of the Si-containing substrate layer 14 of theSOI substrate 12 is inconsequential to the present invention.

The initial SOI substrate 12 can be formed using a layer transferprocess such as, a bonding process. Alternatively, a technique referredto as separation by implanted oxygen (SIMOX) wherein ions, typicallyoxygen, are implanted into a bulk Si-containing substrate and then thesubstrate containing the implanted ions is annealed under conditionsthat are capable of forming a buried insulating layer, i.e., thick BOX16, can be employed.

Next, and as shown in FIG. 2B, at least one trench 26 that extends tothe upper surface of the Si-containing substrate layer 14 is formed bylithography and etching. The lithography step includes applying aphotoresist to the surface of the SOI substrate 12, exposing thephotoresist and developing the exposed photoresist using a conventionalresist developer. The etching step used in forming the trench 26includes any standard Si directional reactive ion etch process. Otherdry etching processes such as plasma etching, ion beam etching and laserablation, are also contemplated herein. The etch can be stopped on thetop of the thick BOX 16 (not shown), or on the Si-containing substrate14 underneath the thick BOX 16, as shown in FIG. 2B. As shown, portionsof the SOI layer 18 and the thick BOX 16 that are protected by thepatterned photoresist are not removed during etching. After etching, thepatterned photoresist is removed utilizing a conventional resiststripping process.

An isotropic oxide etch selective to silicon (such as a timedhydrofluoric acid based etch or similar etch chemistry) is then used toremove portions of the thick BOX 16 underneath the SOI layer 18 wherethe vertical bipolar device will be fabricated (See FIG. 2C). Theisotropic etch forms an undercut 20 beneath the SOI layer 18 that willbe subsequently filled with a conductive back electrode material. TheSOI layer 18 is supported by portions of the thick BOX 16 that are notremoved by this etch. Before this etching step, all pad layers should beremoved from atop the SOI layer otherwise bending of the SOI layeroccurs.

A thermal process such as a wet and/or dry oxidation, nitridation oroxynitridation, is then used to grow the second buried insulating layer22, i.e., thin BOX, on the exposed surfaces of the SOI layer 18, seeFIG. 2D. Note that the second buried insulating layer (hereinafter thinBOX) 22 forms on the exposed horizontal and vertical surfaces of the SOIlayer 18 as well as the exposed surface of the Si-containing substratelayer 14. The thin BOX 22 formed on the SOI layer 18 is given thereference numeral 22 u, while the BOX formed on the Si-containingsubstrate layer 12 is given the reference numeral 221. In accordancewith the present invention, the thin BOX 22 has a second thickness thatis less than the first thickness of the first buried insulating layer,i.e., thick BOX 16. Typically, the thin BOX 22 has a thickness fromabout 1 to about 15 nm. Deposited oxides such as a low-temperature oxide(LTO) or a high-density oxide (HTO) can also be employed. When depositedoxides are used, the oxide would also be present on the sidewalls of theopened structure as well. Note that the oxide also grows, although to alesser extent, on oxide surfaces as well. The growth of oxide on anoxide surface is not, however, differentiated in the drawings of thepresent application.

At this point of the present invention, a conductive back gate electrodematerial (which becomes the conductive back electrode 24) such as, forexample, doped polysilicon, a silicide or a conductive metal isdeposited to fill in the area previously occupied by the removed thickBOX 16. The deposition is performed using a conventional depositionprocess such a chemical vapor deposition, plasma-assisted chemical vapordeposition, chemical solution deposition, evaporation and the like. Inone embodiment, doped polysilicon is used as the conductive backelectrode material and it is deposited at a temperature from about 400°to about 700° C. using a low-pressure chemical vapor deposition (LPCVD)process. Doping of the polysilicon layer may occur in-situ or afterdeposition using an ion implantation process. The structure can then beplanarized, if needed, by chemical mechanical polishing or by a dry etchof the polysilicon selective to oxide. The resultant structure that isformed after performing the above steps is shown, for example, in FIG.2E.

FIG. 3A and FIG. 3B show an SEM cross section of an SOI wafer thatunderwent the process described above. The BOX was undercut by 0.3microns. An 8 nm thick thermal oxide was then grown followed by LPCVDpolysilicon fill.

FIG. 4 shows an expanded cross sectional view of the structure depictedin FIG. 2E. Region 102 denotes the active device area in which a bipolartransistor can be formed. The active area 102 includes an upper thin BOX22 u located atop the conductive back electrode 24. The conductiveelectrode 24, in turn, is located on the lower thin BOX 221, which islocated atop the Si-containing substrate layer 14.

After providing the structure shown in FIG. 2E (or FIG. 4), a bipolardevice such as shown in FIG. 1B is formed atop the structure utilizingconventional BiCMOS processing techniques that are well known to thoseskilled in the art. Specifically, the following process can be used informing the bipolar transistor atop the structure shown in FIG. 2E (orFIG. 4).

First, trench isolation regions 28 are formed into the structure shownin FIG. 2E (or FIG. 4) utilizing conventional processes well known tothose skilled in the art. For example, the trench isolation regions 28can be formed by trench definition and etching, optionally lining thetrench with a liner material and then filling the trench with a trenchdielectric material such as, for example, tetraethylorthosilicate (TEOS)or a high-density oxide. The trench dielectric material can be densifiedafter the filling of the trench and, if needed, a planarization process,such as chemical mechanical polishing, can be employed.

Next, the SOI layer 18 can be further doped at this point of the presentinvention with a first conductivity type dopant (n- or p-type) usingvarious masked implantation schemes to provide an extrinsic collectorand/or intrinsic collector within the SOI layer 18. A first insulator30, such as an oxide, nitride, oxynitride or multilayers thereof, isthen formed on the surface of the structure by a thermal process or bydeposition, such as chemical vapor deposition. The thickness of thefirst insulator 30 can vary depending on the technique used in formingthe same. Typically, the first insulator 30 has a thickness from about10 to about 100 nm.

After forming the first insulator 30 on the surface of the structureshown in FIG. 2E (or FIG. 4), the first insulator 30 is patterned toprovide an opening that exposes a surface of the SOI layer 18. The atleast one opening in the first insulator 30 is formed by lithography andetching.

Next, the base 100 is formed by utilizing a low-temperature epitaxialgrowth process that is typically performed at a temperature from about450° C. to about 800° C. In accordance with the present invention, thebase 100 is comprised of a second semiconductor layer that can include,for example, Si, SiGe or combinations thereof. The low-temperatureepitaxial process forms a base 100 that comprises an intrinsic portion100 a that is typically monocrystalline, and an extrinsic portion 100 bthat is typically polycrystalline. The area in which the material ofbase 100 changes from monocrystalline to polycrystalline is referred toas a facet region.

The base 100 can be doped during the epitaxial growth process or it canbe doped after utilizing ion implantation. An annealing step can be usedto activate the dopants within the base layer. The base 100 is dopedwith a second conductivity type dopant that differs in conductivity typefrom that of the SOI layer 18, which is comprised of a firstsemiconductor material.

The base 100 has a variable thickness in which the extrinsic portion 100b is thickener than the intrinsic portion 100 a. On average, the base100 typically has a thickness from about 10 to about 150 nm.

A second insulator 36 that may comprise the same or different insulatoras the first insulator 30 is then formed on top of the base 100. Thesecond insulator 36 is formed by a conventional deposition process suchas chemical vapor deposition. The thickness of the second insulator 36may vary depending on the process used in forming the same. Typically,the second insulator 36 has a thickness from about 50 to about 150 nm.

The second insulator 36 is then patterned by lithography and etching toprovide at least one opening that exposes the surface of the underlyingintrinsic portion 100 a of the base 100.

Next, the emitter 52 comprising a third semiconductor layer, such as Si,SiGe or a combination thereof, is formed on the second insulator 36 aswell as the exposed surface of the intrinsic portion 100 a of the base100. In accordance with the present invention, the emitter 52 includesthe same conductivity type dopant as the SOI layer 18. The doping of theemitter 52 may occur in-situ or post deposition utilizing an ionimplantation process. The emitter 52 formed typically has a thicknessfrom about 50 to about 200 nm.

The emitter 52 and portions of the second insulating layer 36 are thenpatterned by lithography and etched providing the structure shown inFIGS. 1B and 1C. After patterning, the exposed surfaces containing Si,i.e., extrinsic base portion 100 b, conductive back electrode 24 andemitter 52, are then subjected to a conventional silicidation process inwhich a silicide metal such as Ti, Ni, Co, W, Re or Pt (singularly oralloys thereof) is first deposited and then annealed to causeinteraction of the metal and Si and subsequent formation of a silicideon each region including metal and Si. Alloys of the above mentionedmetals are also contemplated herein. Any remaining metal, not silicided,is typically removed after the silicide process using a conventional wetetching process. It is noted that the silicides formed in the extrinsicportion 100 b of the 100 are self-aligned to the base emitter 52. Thesilicidation process is not shown in the drawings the present invention.

At this point of the present invention, an optional barrier materialsuch as a nitride can be formed atop the structure. The optional barriermaterial is not shown in the drawings of the present invention.

An interconnect dielectric such as, for example, boron phosphorus dopedsilicate glass, an oxide, an organic polymer or an inorganic polymer isthen deposited using a conventional deposited process such as chemicalvapor deposition, plasma-assisted chemical vapor deposition,evaporation, spin-on coating, chemical solution deposition and the like.The interconnect dielectric has a thickness after deposition that is onthe order of about 500 to about 1000 nm. After deposition of theinterconnect dielectric, the interconnect dielectric is planarized bychemical mechanical polishing or other like planarization process so asto have a thickness after planarization from about 300 to about 600 nmand thereafter a contact opening that extends to the surface of eachsilicide is formed by lithography and etching. Each of the contactopenings is then filled with a metal contact such as W, Cu, Al, Pt, Au,Rh, Ru and alloys thereof. The formation of the interconnect structureis not shown in the drawings.

The structure can now be biased by applying an external voltage to theconductive back electrode 24 through the contacts produced above. Thebiasing causes an accumulation layer 62 to be formed in a portion of thebase SOI layer 18 that is located above the thin BOX 22 u. The amount ofvoltage applied in forming the accumulation layer 62 is typically 5 V orless. The accumulation layer 62 serves as the subcollector of theinventive structure.

The method described above can be used to form a plurality of verticalbipolar transistors on the active area of the SOI substrate. The methodsdescribed above can also be used in conjunction with a conventional CMOSprocess flow which is capable of forming CMOS devices such as fieldeffect transistors, in areas adjacent to the areas containing thevertical bipolar transistors of the present invention, to form BiCMOSfor RF or mixed-signal applications. In the prior art, the CMOS devicesare typically formed prior to the bipolar devices with CMOS areasusually protected during fabrication of the bipolar transistors. Thedrawback of this method is that the MOS device performance often becomesdegraded to the excessive thermal budget that CMOS devices experienceduring the fabrication of the dipolar devices, such as dopant activationanneal after implants. An advantage of this invention over prior artprocesses is that the inventive method utilizes the typical CMOS processto form a bipolar device hence the CMOS and bipolar devices can befabricated interactively and share the same activation anneal.

It is noted that the processing steps described above in forming thesubstrate shown in FIGS. 2A-2E and FIG. 4 can be used in conjunctionwith conventional back-gate processes to form a back-gated CMOS device.The CMOS device can be provided separately from the SOI BJT devicedepicted in FIG. 1A and FIG. 1B, or it can be formed together with theSOI BJT device or any other bipolar transistor in a BiCMOS application.

Specifically, FIG. 5A and FIG. 5B shows a back-gated CMOS device thatcan be formed on the substrate shown in FIG. 2A or FIG. 4. Theback-gated device 300 includes Si-containing substrate layer 14, a firstburied insulating layer 16 having a first thickness located atop aportion of the Si-containing substrate 14. Other portions of theSi-containing substrate include the lower portion 221 of the secondburied insulating layer 22 having a second thickness that is less thanthe first thickness. A back-gate electrode 24 is located atop the lowerportion 221 of the second buried insulating layer 22. An upper portion22 u of the second buried insulating layer 22 is located atop theconductive back electrode 24 and portions of first buried insulatinglayer 16. SOI layer 18 including source/drain diffusion regions 310 andextension regions 320 is located atop the upper portion 22 u of thesecond buried insulating layer. Trench isolation regions 28 which extendto the first buried insulating layer 16 are located abutting the SOIlayer 18. The front-gated device also includes, a gate dielectric 300located atop a portion of the SOI layer 18 and a gate conductor 350located atop the gate dielectric 350. An optional, but preferred,reoxidation liner 340 is present on at least the sidewalls of the gateconductor 350. Insulating spacers 330 are located in the structure aswell. Gate dielectric 300 and gate conductor 350 are components of afield effect transistor.

The back-gated device 300 is fabricated by first utilizing themethodology used in forming the substrate shown in FIGS. 2E and 4, andthen by utilizing conventional CMOS processes steps that are well knownin the art. The gate dielectric 300, the gate conductor 350 and theinsulating spacers 330 are composed of conventional materials well knownto those skilled in the art. For example, the gate dielectric 300 can bean oxide, nitride, oxynitride, a dielectric material having a dielectricconstant greater than 4.0, preferably greater than 7.0, or a stackthereof; the gate conductor 350 can be composed of polySi, polySiGe, ametal, a metal silicide, a metal nitride or any combination, includingmultilayers thereof, and the insulating spacers 330 are composed of anoxide, nitride, oxynitride or combinations, including multilayersthereof.

While the present invention has been particularly shown and describedwith respect to preferred embodiments, it will be understood by thoseskilled in the art that the foregoing and other changes in forms anddetails may be made without departing from the spirit and scope of thepresent invention. It is therefore intended that the present inventionnot be limited to the exact forms and details described and illustrated,but fall within the scope of the appended claims.

1. A method of fabricating a bipolar transistor comprising the steps of:providing a silicon-on-insulator (SOI) substrate comprising a firstsemiconductor layer containing a first conductivity type dopant locatedover a first buried insulating layer, wherein a portion of the firstburied insulating layer beneath said first semiconductor layer isremoved providing an undercut region; forming a second buried insulatinglayer on exposed surfaces of said first semiconductor layer, whereinsaid second buried insulating layer is thinner than said first buriedinsulating layer; filling the undercut region and the removed portion ofthe first semiconductor layer with a conductive back electrode material;forming a base comprising a second semiconductor layer containing asecond conductivity type dopant that is different than the firstconductivity type dopant; forming an emitter comprising a thirdsemiconductor layer including said first conductivity type dopant over aportion of said base; and biasing the conductive back electrode materialto form an accumulation layer at an interface between the firstsemiconductor layer and the second buried insulating layer.
 2. Themethod of claim 1 wherein said providing the SOI substrate comprisesforming a trench into the first semiconductor layer, stopping on saidfirst insulating layer; and performing an isotropic etch process to formsaid undercut region.
 3. The method of claim 1 wherein said forming thesecond insulating layer comprises a thermal growth process.
 4. Themethod of claim 1 wherein said filling the undercut region comprisesdepositing a doped polysilicon layer.
 5. The method of claim 1 whereinsaid base is formed utilizing an epitaxial growth process that iscapable of forming polycrystalline semiconductor material atop aninsulator and single crystal semiconductor material a top asemiconductor material.